Re: [SI-LIST] : Schottky diode termination

Dennis Tomlinson (det@tellabs.com)
Wed, 28 Oct 1998 11:06:42 -0600

Andrew Ingraham wrote:
>=20
> John,
>=20
> I first saw Schottky diode termination described in a rather old
> Fairchild ECL book. Diode termination was not often used for ECL,
> perhaps because the normal ECL parallel termination usually worked quit=
e
> well if you could afford the power dissipation.
>=20
> The TTL logic families had clamp diodes in their input structures, and
> one of their intended functions actually was to provide some amount of
> diode termination. This is one of the reasons why TTL was easy to work
> with, sometimes even when the wires got long compared to the falltimes.
> Many people who have used TTL do not realize that they have been making
> use of diode termination all along.
>=20
> Most CMOS families also have input clamp diodes, and again they
> generally work as both input protection and partial signal termination.

Andrew, John, list,

It has long been my belief that the input clamp diodes on CMOS are for
the purpose of providing ESD protection, and for clamping the "occasional=
"
over/undershoot. They are not provided for the purpose of providing line
termination. My belief is based on verbal communiqu=E9 with cohorts and
apps. engineers (which is to say, it could be based on legend and=20
mis-information).

As a crude example, suppose a full 5V transition is launched onto an
electrically long 50 Ohm line, causing the propagation of a 100 mA=20
current wave. With no other termination or loads, and assuming a clamp=20
forward bias of about 1V, this would require the clamp diode to conduct=20
a peak current of about 80 mA. If this were a highly repetitive signal,=20
the duty cycle for each diode could approach 50%. I would expect serious=20
MTBF issues with this scenario. I would also expect the semiconductor
types on this list to shudder at the thought of such an abusive applicati=
on=20
(aside from the strong possibility that the circuit may not function corr=
ectly).

I would like to pose some related questions for anyone on the list - most
particularly the semiconductor types:

1. Do the semiconductor companies ascribe and/or acquiesce to the use of =
CMOS
input clamp diodes for line termination?

2. What are typical limits for "one time" peak currents?

3. What are typical limits for repetitive peak currents?

4. What if I were to change the example above such that the driver launch=
ed a=20
3.5 volt wave (70 mA)? The clamp current would then peak at roughly 20=
mA.

5. And finally, what if the line length were shorter, so that the clamp
duty cycle were only a few percent?

>=20
> Those CMOS devices without input diodes tend to be SI nightmares.
>=20
> In a multi-drop bus configuration, diode terminators may be needed at
> several places, perhaps as many as one per input. For a daisy-chain
> route, they may only be needed at the two ends. The best way to tell
> what works is to try, preferably in simulation with GOOD models.
>=20
> Regards,
> Andy Ingraham
>=20

TIA,

Dennis

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